Field of the Invention
The invention relates to a biochip device for analyzing biological molecules by fluorescent marking.
Description of the Related Art
In such devices, a substrate includes pads constituted by probe molecules capable of hybridizing in preferential manner with target molecules contained in a hybridizing solution obtained from a sample to be analyzed. The target molecules are marked with the help of chromophore elements capable of emitting fluorescence with they are excited by appropriate light, the wavelength of the fluorescence depending on the nature of the chromophore elements.
After hybridizing, the biochip is dried and illuminated with a light source at the excitation wavelength of the chromophores marking the target molecules, and an image of the fluorescence of the biochip is picked up with the help of appropriate objects. In the image obtained in this way, the intensity of each point is associated with the quantity of chromophores present at the corresponding point of the biochip and thus associated with the number of target molecules that have been selectively fixed at that point during the hybridizing stage thus making it possible to obtain information about the biological species content of the hybridized solution.
That type of sequential reading of the fluorescence of the biochip after hybridizing is nevertheless unsuitable for performing real time reading of the hybridizing signal since the stages of hybridizing and of image taking are spaced apart in time, and take place in separate hybridizing and reading appliances.
Certain appliances are capable of performing both the hybridizing and the reading stages, thus making it possible to detect the signal in real time during the hybridizing stage (see in particular Y. Marcy, P.-Y. Cousin, M. Rattier, G. Cerovic, G. Escalier, G. Béna, M. Guéron, L. McDonagh, F. L. Boulaire, H. Benisty, C. Weisbuch, J.-C. Avarre, “Innovative integrated system for real time measurement of hybridization and melting on standard format microarrays” Biotechniques 44, 2008, 913). The image of the fluorescence of the pads carrying the hybridized molecules is acquired in the presence of the hybridizing liquid containing the target molecules that are marked, and thus fluorescent, and they may be present at high concentration. Fluorescence is then observed coming simultaneously from the target molecules attached to the pads of probe molecules (forming the useful signal) and from fluorescent molecules in the solution (constituting a background signal that is added to the useful signal).
That is disadvantageous, since the strong background signal generated by the fluorescent species in solution limits the sensitivity with which it is possible to detect the attachment of target molecules and limits the dynamic range over which hybridization can be measured.
In order to avoid that drawback, one possibility consists in selectively exciting the molecules at the surface of the biochip without exciting the molecules present in the solution, by using an evanescent wave at the surface of the biochip so as to excite only the fluorescent pads (one technique often used for that purpose is a configuration of the total internal reflection fluorescence (TIRF) type). By way of example, other evanescent wave excitation methods consist in using substrates carrying a waveguide, preferably a monomode waveguide, and in exciting one or more modes in the waveguide with the help of etched coupling gratings or in exciting guided modes in the biochip by lighting via an edge face (US 2004/077099 A1).
In general, it is also necessary to take into consideration the interaction between the guided waves and the optofluidic portion of the device in contact with the waveguide.
For the above-described evanescent wave devices, light coupling makes it necessary to use excitation devices having mechanical constraints that are very demanding in terms of precision.
That type of coupling makes it necessary either to use optical systems with sub-micrometer precision on polished edge faces for coupling the excitation light to a single mode, or else to have recourse to beams that are collimated with very precise angles (a few milliradians or less).
Nevertheless, it is known that incident light on non-uniform bodies such as metallic or dielectric particles, or more generally diffusers, make it possible to excite guided modes of any planar structure providing the elements of the diffuser are positioned very close to the waveguide, in the evanescent tail of the modes. This makes it possible to avoid the tight coupling tolerances encountered with the above-described devices. Such diffusers are referred to herein as “substantially non-directional means for generating or coupling guided modes”.
The term “substantially non-directional coupling means” is used herein to designate means for coupling excitation light into the waveguide in the form of waves that are guided in a plurality of directions inside the waveguide by using excitation light coming from a plurality of directions. The excitation light may be coupled with the waveguide by using an excitation light beam that is not necessarily collimated. With such coupling means, there is no longer any need for the beam to be oriented very precisely relative to the waveguide.
Such coupling means are known for waveguides and solar cells, e.g. made of silicon. In those applications, a diffusing disordered interface serves to transform the incident light into guided light so that it is used in the waveguide or absorbed in the solar cell. For waveguide applications, the purpose is then to use the light in the waveguide so that it is absorbed therein, e.g. for use in a photodetector device. For solar cells, diffusion takes place over the entire surface of the cell in order to be able to capture all of the light intercepted by the cell.
An object of the invention is to provide a simple solution to the above-mentioned problems of biochip devices known in the prior art.
To this end, the invention provides a biochip device comprising a substrate constituted by at least one plate of material forming a multimode waveguide and carrying chromophore elements suitable for emitting fluorescence in response to excitation by guided waves having an evanescent portion, the device being characterized in that it includes coupling means for coupling excitation light with the waveguide in the form of guided waves, the coupling means being substantially non-directional.
Integrating substantially non-directional coupling means in a biochip device makes it possible to avoid the precision constraints encountered in the prior art.
In the invention, the coupling means cover only a portion of the biochip. In particular, the coupling means are placed at a distance from the fluidic or optofluidic portion so as to avoid extracting guided waves into the fluid containing fluorescent molecules, which is precisely what it is sought to avoid by exciting fluorophores that are excited by the evanescent waves only.
In an advantageous configuration, the device includes mode filter means for eliminating from the waveguide guided modes having an effective index less than or equal to a predetermined threshold value, this threshold value being selected so that no guided mode escapes from the waveguide beyond the zone having the mode filter means.
A first drawback of approaches based on substantially non-directional coupling means lies in the low efficiency of the coupling of the exciting modes with the guided modes. In order to reach a given guided mode intensity, it is possible to use an exciting source that is more intense. Nevertheless, the main drawback with a multimode waveguide is that that type of method of exciting guided modes tends to excite modes regardless of their effective index. Unfortunately, modes with smaller effective indices correspond to modes that leave the waveguide and penetrate into the fluid or into the optofluidic portions, where they contribute to increasing the background signal.
Because the guided modes transfer a propagating flux into the fluid only on contact with the fluid or the optofluidic portion, the use of non-directional coupling means can advantageously be combined with mode filter means that eliminate the unwanted modes that are capable of interacting with the fluid or the optofluidic portion.
From a theoretical point of view, the condition for non-transfer of a guided mode to an interface is conventionally presented in the form of an angle (angle of incidence at the interface being greater than a critical angle), however in more fundamental terms this condition can be expressed in the form of an effective index of the guided mode, which effective index must be greater than that of the fluid or of the optofluidic portion.
Generalizing from the above propositions, in the device of the invention, easy mechanical coupling is provided by means for generating guided waves that are low directional, such as optical diffusing media in particular, whereas the mode filter means serve to selectively filter out those of the guided modes that can be extracted from the waveguide and thereby increase the interfering background signal. Thus, the guided modes of effective index that is less than that of the material of the hybridizing chamber and than that of the biological solution are filtered out before they reach the optofluidic zone and a fortiori before they reach the zone carrying the chromophore elements, thereby avoiding exciting free chromophore elements in solution and out of reach of the evanescent wave.